1. Field of the Invention
This invention relates to a superlattice semiconductor structure in which doped impurities are activated at a high rate.
2. Disclosure of the Prior Art
The conventional method for doping impurities into compound semiconductors distributed uniformly impurities in the compound semiconductors having a uniform structure, similar to the doping of impurities into silicon (Si) or germanium (Ge). The conventional distribution of impurities in the compound semiconductors was uniform over the entirety of the semiconductor material. While some compound semiconductors such as GaAs and InP may achieve a high electron or hole density by the conventional techniques, other compound semiconductors can not obtain a high electron or hole density.
For example, for doping silicon as impurities into Al.sub.x Ga.sub.1-x As (x=0.2.about.0.6), the conventional technique grew Al.sub.x Ga.sub.1-x As containing Si as impurities on a GaAs substrate by a molecular beam epitaxy (MBE) process. If, however, the doping density of Si was made at 1.times.10.sup.18 cm.sup.-3 under a condition that the GaAs substrate was held at a temperature of 580.degree. C., the activated electron density in the grown Al.sub.0.3 Ga.sub.0.7 As layer was at most 1.times.10.sup.17 cm.sup.-3. This activated electron density could be increased to a value of 3.times.10.sup.17 cm.sup.-3, if the substrate temperature was raised at 730.degree. C., but it was impossible to increase the electron density furthermore. This is based on the coexistence of Al and Ga atoms around the impurities in the semiconductor layer. Due to the coexistence of Al and Ga atoms, even if the impurity doping density is increased further, lattice defects increase to compensate the increase of impurity density. The similar phenomenon was observed in cases where impurities were doped into GaAsP, GaAsSb or a mixture of Si and Ge. The coexistences of As and P, As and Sb and Si and Ge may make the number of impurity-related lattice defects increase to compensate the increase of impurity density.
An upper limit of hole density was also observed when acceptor impurities were doped into II-VI semiconductors having a band gap wider than 2.0 eV. Such upper limit was very low. Therefore, while the II-VI semiconductors could have a property of N-type conductivity, they could not have a property of P-type conductivity.